Towards a Mechanistic Understanding of Moisture Damage in Asphalt Concrete

Recently, dynamic modulus ($|E^{*}|$) test has gained substantial acceptance for Superpave mix performance evaluation and in designing asphalt pavements for rutting and fatigue cracking. Mechanistic empirical pavement design guide (MEPDG currently branded as AASHTOW are ME pavement design) is currently the state-of-the-art method of designing asphalt pavements and it utilizes $|E^{*}|$ data. Unfortunately, neither Superpave mix design method nor ME pavement design has used $|E^{*}|$ to evaluate asphalt concrete (AC) moisture damage. Though $|E^{*}|$ based rut and fatigue models are available in ME pavement design, there has not been any model for moisture damage of AC. To this end, this study attempts to correlate $|E^{*}|$ with moisture damage as a first step to moving toward mechanistic understanding. Moisture-induced sensitivity tester (MIST) is used for moisture conditioning of asphalt samples. Moisture damage resulting from MIST conditioning is evaluated using the dynamic modulus ratio (DMR) of wet to dry samples. MIST conditioning is found to decrease the $|E^{*}|$ value of asphalt concrete (AC) indicating the presence of moisture damage. Moreover, moisture damage shows an increasing trend with increasing MIST-conditioning number of cycles, temperatures, and pressures. To capture the correlation between DMR with MIST-conditioning cycles and pressures, two different models are developed. The coefficient of determination $R^2=0.969$ and $R^2=0.839$ are obtained for the MIST-cycle and MIST-pressure models. It is hoped that these models are incorporated in the ME pavement design in future.